Electrophotographic method and member for contact printing of relatively opaque documents



y 9, 1970 c. E. HERRICK, JR 3,512,967

ELECTRQPHOTOGRAPHIC METHOD AND MEMBER FOR CONTACT PRINTING OF RELATIVELYOPAQUE DOCUMENTS Filed Nov. 9, 1966 3 Sheets-Sheet 1 [NT/ENTOR CLIFFORDE. HERRICK,Jr.

BYW U. m

ATTORNEY May 19, 1970 c. E. HERRICK, JR 3,512,957

ELECTRQPHOTOGRAPHIC METHOD AND MEMBER FOR CONTACT PRINTING OF RELATIVELYOPAQUE DOCUMENTS Filed Nov. 9, 1966 3 Sheets-Sheet 2 IMAGE CONTRAST GAINl 0.00 2.0 '60 0.40 F|G.3 1.0 I 0.50

FRACTIONAL UGHT ABSORPTION IMAGE 31 A CONTRAST GAIN 20 0.00

FRACTIONAL LIGHT ABsORPTION 000 IMAGE I. CONTRAST 1' I (SAW l i t 0.

2.0- 1 "1' +-1I-- l I i I I I i F|G.5 1.0 E! I 1 1 L FRACTIONAL LIGHTABSORPTION United States Patent 3,512,967 ELECTROPHOTOGRAPHIC METHOD ANDMEM- BER FOR CONTACT PRINTING OF RELATIVELY OPAQUE DOCUMENTS Clifford E.Herrick, Jr., Los Gatos, Calif., assignor to International BusinessMachines Corporation, Armonk, N.Y., a corporation of New York Filed Nov.9, 1966, Ser. No. 593,051 Int. Cl. G03g 13/22 US. Cl. 96-1 ClaimsABSTRACT OF THE DISCLOSURE A contact printing electrophotographic methodin which an opaque document such as bond paper is brought intoface-to-face contact with a photoconductive layer on a highly lightreflective backing, followed by uniformly exposing the photoconductivemember through the back of the document. The exposing light is reflectedback and forth between the highly light reflecting backing of thephotoconductive member and the nonimage face surface of said documentwith only a per tion of the light being absorbed by the photoconductivelayer during each passage of light, thereby enhancing the image contrastof the resultant copy of the document.

This invention relates to an electrophotographic method and member and,more specifically, relates to the exposure of the electrophotographicmember.

In the method of electrophotography, commonly known as xerography, aphotoconductive member is given a uniform electrostatic charge over itssurface and is then exposed to an opaque document to be reproduced by aconventional projection technique, such as scanning an illuminateddocument with a photographic copying camera having a linear opticalsystem. An image of the document to be reproduced is reflected from thedocument as a light image which is transmitted through an objective lensand onto the photoconductive surface. The areas of the photoconductivemember exposed to the light image are discharged and an electrostaticlatent image is created. Development of this electrostatic latent imageis then achieved with an electrostatically charged material, such aselectroscopic powder which is brought into surface contact with thephotoconductive member and is held thereon electrostatically in apattern corresponding to the electrostatic image. Thereafter, thedeveloped electrostatic image is transferred to a surface, such aspaper, to which it may be fixed by heating or any other suitable means.

Exposure of the photoconductive member by the projection system, asdescribed above, is disadvantageous for a number of reasons. Forexample, lenses of good optical quality are expensive and theirelimination can result in appreciable savings. Moreover, due to thephotographic speed of present day photoconductors, projection exposuresin most cases require exposures of a few seconds up to 20 or 30 seconds.Further, with a projection exposure system, the document must bepositioned away from the photoconductive member at a distance determinedby the focal length of the lens system, thereby increasing the spacerequirements and, consequently, the flexibility of the design of theelectrophotographic apparatus.

An alternative exposure system in which the photoconductive member isexposed Without a lens system is contact printing. In this type ofsystem, the photoconductive member is brought into contact with thedocument to be reproduced and is exposed to a light source through thedocument. One of the principal advantages of contact printing is that itis many times more efficient in it utilization of light than aprojections system and, hence, is much better suited for high speedoperation. However, in order to obtain a good quality reproduction bythis system with photoconductors such as selenium, the document must betransparent or translucent to the exposing radiation and carry a dense,high contrast image. Otherwise, the contrast ratio of the image tononimage areas will not be suflicient to provide a developableel'ectrostatic image of good quality.

Accordingly, it is an object of the present invention to provide a newand improved electrophotographic method employing contact printing andyielding high quality reproduction of opaque documents.

It is another object of the present invention to provide a new andimproved electrophotographic member which enhances the image contrastwhen employed with opaque documents.

Still another object of the present invention is to provide a new andimproved electrophotographic apparatus which is inexpensive, compact insize, and capable of operating at a high rate of speed. a

In general, the foregoing and other objects and other advantages of thepresent invention are achieved by an electrophotographic method in whichan opaque document is brought into face-to-face contact with aphotoconductive member comprising a photoconductive film and with ahighly reflecting backing, and exposing through the back of the documentto light which is only weakly absorbed by the photoconductive film andto which the photoconductive film is substantially transparent.

Other and further objects and advantages of the invention will beapparent in the following more particular description of the preferredembodiments of the invention, as illustrated in the accompanying drawingin which:

FIG. 1 represents an electrophotographic apparatus embodying theexposure system of the present invention.

FIG. 2 is an enlarged fragmentary cross-section of the photoconductivemember of the exposure system of FIG. 1.

FIGS. 3, 4, and 5 are graphs to be utilized in explaining the mechanismof the exposure technique of the present invention.

FIGS. 6, 7 and 8 are graphs to be utilized to show the improvements inimage contrast with the photoconductive member of the present invention.

The electrophotographic apparatus shown in FIG. 1 embodying the exposuresystem of the present invention comprises a rotatable drum 1 carryingaround its periphery an electrophotographic photoconductive member 2having situated adjacent its periphery a corona unit 3 forelectrostatically charging the surface of the drum. Herein, the drum 1rotates in a clockwise direction and after its surface iselectrostatically charged by the corona unit 3, a document 5 carrying animage 6 to be reproduced is fed into virtual contact with theelectrostatically charged surface past the exposure station 7 whereinthe photoconductive member 2 is illuminated through theback 8 of thedocument 5 by a light source 9 having an exposure slit 10. Theelectrostatic charges on the area of the photoconductive member 2corresponding to the nonimage areas of the document 5 are dissipated toa greater degree than the electrostatic charges on the areas of thephotoconductive drum corresponding to the image areas, thereby forming adevelopable electrostatic image. After exposure, the document 5 isseparated from the drum 1 and the drum passes a development station 11at which a toner-carrier mixture 12 is gravity fed across theelectrostatic image on the surface of the photoconductive member 2. Thetoner having a charge opposite from the polarity of the electrostaticimage is attracted to the image rendering it visible. Herein, an endlessconveyor belt 13 carries the toner-carrier mixture to a position forgravity feeding it across the surface of the photoconductive member.Continuing the clockwise rotation of the drum 1, a copy paper 14 is fedinto contact with the developed electrostatic image. Preferably, acorona unit 15 is disposed beneath the paper at the area of contact andwith a polarity opposite that of the toner, thereby attracting the tonerto the copy paper. After this so-called corona transfer, the paper 14 isseparated from the drum and fed past a fusing element 20 to permanentlyfix the toner to the paper. The drum 1 continues to rotate past acleaning brush 16 which wipes the surface of the photoconductive memberso as to remove any excess toner. This completes a cycle of the drum.

In accordance with the present invention, the photoconductive membercomprises a film of weakly light absorbing photoconductive materialbeing substantially transparent to light and with a highly reflectingbacking so that, in being exposed through an opaque document, the lightpenetrating the document passes through the photoconductive film and isreflected between the highly reflecting backing and the reflectivenonimage areas of the document, two portions of the light being absorbedby the photoconductive material during each reflection cycle. In theimage areas, however, the light reflected from the backing of thephotoconductive material is absorbed rather than being reflected by theimage. Therefore, there is a substantial increase in the ratio of lightabsorbed in the nonimage areas to the light radiation absorbed in theimage areas as compared to the ratio achieved with a strongly absorbingphotoconductive material.

As best shown in FIG. 2 for purposes of illustration, thephotoconductive member 2 comprises a photoconductive film 17 and abacking 18, both of which are carried on the drum 1. The exposing light,as shown by the arows 19, strike the back of the opaque document and aportion of the light rays are reflected off of the document dependingupon the reflectivity, R,, of the back surface 8 of the document. Theremainder of the light rays pass through the document with a portionbeing absorbed depending upon the transmission density, D,,, of thedocument. Another portion of the light rays in the image area 6 areabsorbed by the image depending upon its transmission density, D At thispoint, if the photoconductor would totally absorb the remaining lightstriking its surface, the exposure contrast, C (i.e.the ratio of lightabsorbed in the photoconductive material in the nonimage areas to thelight absorbed in the image areas) is essentially dependent only uponthe transmission density of the image area, D.

However, in accordance with the present invention, a weakly absorbingphotoconductive material or one which only absorbs a fraction of thelight striking its surface is employed. Hence, in addition to D theexposure contrast, C becomes dependent on the fractional lightabsorption, A, of the photoconductive material; the reflectivity, R ofthe backing of the photoconductive material; and the reflectivity, R,,',of the face surface in the nonimage areas of the document. That is, inthe process of the present invention, the initial light striking thephotoconductor which is not absorbed by it passes through thephotoconductor and is reflected back from the backing of thephotoconductor. Again, a portion of the light is absorbed and theremaining portion passes out of the photoconductor. This portion,depending on whether an image or nonimage areas is adjacent thephotoconductor, either is reflected back from the surface of thenonimage area of the document or is substantially absorbed by the image.The document reflected light passes through the photoconductor, with aportion being absorbed, and is reflected back oif of the backing of thephotoconductor. Also, a portion of this light is absorbed by thephotoconductor as it travels back toward the document.

Thus, it can be seen from the foregoing that the light initiallyreaching the photoconductor is reflected back and forth between thebacking of the photoconductor and the nonimage surface of the document,as shown in FIG. 2. During each complete reflection cycle, two portionsof the light are absorbed by the photoconductor. In addition, in theimage areas of the document adjacent the photoconductor, each time aportion of the light is reflected out of the photoconductor, a majorportion of it is absorbed by the image with the amount of the absorptiondependent on the image density, D Even with documents having relativelypoor image density, the amount of light which is not absorbed and,hence, is reflected back towards the photoconductor, is significantlyless than the amount of light reflected from the nonimage surface.

To further illustrates the contrast enhancement or gain of the presentinvention, reference is now made to the graphs shown in FIGS. 3, 4, and5. It will be recalled that, with a photoconductor which totally absorbsthe light striking it, the image contrast, C is only a function of theimage density, D More accurately, C, is equal to 10 1. Conversely, theimage contrast, 0,, in the process of the present invention is also afunction of R R and A, as defined above. Therefore, setting C =1O i asthe reference, the enhancement of image contrast over this reference orgain, 6,, can be expressed as follows:

wherein the terms are defined as stated above. That is, the term, G,, isthe ratio of exposure contrast of the contact exposure method of thepresent invention to the exposure contrast of a contact printing methodwith a strongly absorbing photoconductor.

Turning now to FIG. 3, there is shown a graph, based on the aboveexpression, plotting contrast gain, G versus the fractional lightabsorption of the photoconductor for a document having an imagetransmission density, D of 1.0. The curves are plotted for differentproducts of reflectivity, R R ranging from 0.30 to 0.85. The lower ofthese two reflectivity products is representative of a low reflectivedocument, such as vellum, and the higher numbered reflectivity productrepresents an opaque highly reflective document. It will be seen fromthis graph that the contrast gain increases as the fractional absorptionof light of the photoconductor decreases and the product of reflectivityincreases. This is also shown to be the case when the image transmissiondensity is low, such as 0.2, as shown in the graph of FIG. 4.

In FIG. 5, a graph is shown in which the contrast gain, 6,, is plottedversus the image transmission density for a photoconductor having a 5%light absorption. This graph illustrates that the contrast gain, G,, isconstant over a wide range of image densities. FIG. 5 also amplifies theinfluence of the reflectivity product on the gain, but it will be notedthat, even with low reflective documents (the 0.30 curve), such asvellum, there is an image contrast gain over the use of a strongly lightabsorbing photoconductor.

Because of the importance of the reflectivity product, R R in obtaininghigh quality reproduction of highly opaque documents, the backing of thephotoconductive film must provide a smooth surface at the interface ofthe photoconductive film and the backing so as to be highly reflective.In other words, the reflectivity, R of the backing must be suflicientlyhigh so that the reflectivity product, R R approaches the reflectivity,R of the document. Preferably, the material forming the backing ofphotoconductive layer should have a surface capable of reflectinggreater than of the light in the wavelength range absorbed by thephotoconductor. Ninety percent or greater is more preferred.

Suitable materials for the backing of the photoconductive member of thepresent invention are aluminum, gold, silver, copper, magnesium,calcium, and rhodium, which may be coated, preferably by evaporation, ona suitable substrate of plastic, metal, or paper. Other reflectivematerials can be found on pages 6-104 through 6-110 of the AmericanInstitute of Physics Handbook (1957). In addition, it is preferred thatthe reflective backing material be specular reflecting as well as highlyconductive. For that reason, the backing material herein is aluminumdeposited on a substrate of polyethylene terephthalate. If desired,however, a low conductive or nonconductive backing material can beemployed in conjunction with the dual corona device of U.S. Pat.2,922,883.

The reflectivity, R of the nonimage areas of the document is alsoimportant, but can vary over a wide range from transparent up toextremely highly opaque documents. Preferably, the reflectivity, R ,.ofthe nonimage areas of the document is such that the reflectivityproduct, R R is greater than 0.25. More preferably, the documents shouldreflect greater than 70% of the light in the nonimage areas. If,however, very low reflective documents are to be copied, a highly opaquesheet may be placed in back of the document to form a composite whichhas a greater reflectivity thanthe document itself and provides areflectivity product, R R greater than 0.25.

The photoconductive material of the photoconductive member, must onlyweakly absorb the light to which it is exposed. Herein, thephotoconductive material should only weakly absorb light Within theWavelength range of 4000-6500 A., which is the wavelength range of thepreferred light source. By weakly absorbing, it meant that thephotoconductive material preferably should absorb less than 30% of thelight each time the light passes through the photoconductive film. Inaddition, the photoconductive material should be nonlight scattering sothat totally internally reflected light rays will not be generatedwithin the photoconductive film. Accordingly, organic photoconductorsare the preferred materials for use in the present invention and includewhat are termed small molecule photoconductors dispersed or dissolved inan essentially transparent binder and polymeric photoconductors whichcan be self-supporting. Examples of such organic photoconductors arelisted in copending application, Ser. No. 474,583, now abandoned andrefiled as Ser. No. 847,493 on July 14, 1969, and copending application,Ser. No. 474,977, filed July 26, 1965. As a general rule, thesensitivity of organic photoconductors normally is in the ultravioletregion of the electromagnetic spectrum, but can be extended into thevisible region by the addition of a dyestufl sensitizer. Also,activators can be added for increasing the photoconductivity of thephotoconductor and, in some cases, to shift the sensitivity of thephotoconductor into the visible region. Examples of both dyestuffs andactivators can be found in abovereferenced copending applications asWell as in British Pat. 942,810 and U.S. Pat. 3,169,060.

'If the mode of operation in which the photoconductive member isemployed is one in which the conductive image in the photoconductivematerial should not persist after exposure, suchv as is the case withconventional xerography, then the activators should be selected from thequinones, ketones, and aldehydes listed in the above-referenced patents.If, however, the photoconductive member of the present invention is tobe used in a persistent electrophotographic mode in which the conductiveimage should persist after exposure because, for example, it is exposedprior to charging, then the photoconductive materials of the referencedcopending applications are preferred.

The general nature of the invention having been set forth, the followingspecific examples are now presented as illustrations, but notlimitations, of the present invention. The following examples willinclude a comparison 7 of the image quality of copies prepared using astrongly light absorbing photoconductor and the photoconductive memberof the present invention in which the photoconductor is only weaklylight absorbing. The examples will also show the voltage change (Av.) inthe exposed areas of a strongly light absorbing photoconductor and thephotoconductive member of the present invention.

EXAMPLE I Three photoconductive compositions were prepared and coated onthe aluminum side of three separate films of aluminized polyethyleneterephthalate. One composition contained 1:1 molar ratio ofpoly-N-vinylcarbazole and 2,4,7-trinitro-9-fluorenone and is describedin copending application, Ser. No. 556,983, filed June 13, 1966. Theother compositions contained 40:1 molar ratio and :1 molar ratio ofpoly-N-vinylcarbazole and 2,4,7-trinitro-9- fluorenone. The reflectivityof the aluminum surface for all three photoconductive members was about92% or greater of the incident light in the visible range of theelectromagnetic spectrum. Using a laboratory contact printing device,the prepared photoconductive members were individually tested. Thedevice comprised an electrostatic charging station having a corona unitand a contact exposure station having a l5-watt white fluorescent lightsource positioned to be 1 /2 inches from the document as it passed bythe exposure station. Also, the device included a toning station fordeveloping the electrostatic image by conventional cascade development.

During the testing of the three samples, the exposure setting wasoptimized for each sample because they varied in light sensitivity. Withthe 1:1 molar sample, it was necessary to employ a 1:1 neutral densityfilter over the exposure aperture to limit the light intensity in orderto achieve the optimum exposure. With the 40:1 molar and the 100:1 molarsamples, an orange filter was placed over the aperture to blockwavelengths below about 5000 A. because these wavelengths are morestrongly absorbed. When measured on a Macbeth Ansco densitometer the 1:1molar, 40:1 molar and 100:1 molar samples absorbed approximately 92%,29%, and 20%, respectively, of the light. (Wavelength range of 4000-6500A. for the 1:1 molar sample and 5000-6500 A. for the other two samples.)

All of the films were electrostatically charged to substantially thesame voltage of 600 volts with any variation being due to the thicknessof the film. The document to which the samples were exposed was acomposite of three different documents, one being highly opaque and theother two of low opacity. The print density on the three separatedocuments forming the composite also varied, with the greatest printdensity being on the highly opaque document.

Molar composition of poly-N- vinylcarbazole and 2,4,7-trinitro-Q-fluorenone Quality or copy Original Image Background 1:1 molar. (a)Highly opaque document:

(1) High print density.-- Very good... Fair. (b) Low opacity document:

(1)High print density... Fair (2) Low print density..- Very poor... 40:1molar. (a) Highly opaque ducument:

(1) High print density Very good--- Good. (b) Low opacity document:

Very poor.

(1) High print density.-- Good+. Do.

Low print density... Good- Do.

100:1 molar- (a) Highly opaque document:

(1) High print density..- Very good... Do. (1)) Low opacity document:

(1) High print density.-- do Good+. (2) Low print density-.- Good Go0d+.

conductor is distinctly superior to the strongly light absorbingphotoconductor.

EXAMPLE II Using the 1:1 molar and the 40:1 molar samples of Example I,plus a sample comprising a 150:1 molar ratio of poly-N-vinylcarbazoleand 2,4,7-trinitro-9-fluorenone, the three samples were individuallyexposed through a gray scale step Wedge prepared on black developingdiazo paper. Using the same densitometer of Example I, the absorption ofthe 15021 molar sample was measured and found to be approximately(Wavelength range of 5000-6500 A.). Each sample was exposed at exposuresettings ranging from 3.2 to mm. and again the orange filter was usedfor the 40:1 molar ratio and the 150:1 molar sample, but not the 1:1molar sample, the neutral density being used for the latter. Prior toeach exposure of the 1:1 molar sample, the sample was charged to 600volts and prior to each exposure of the other two samples, those sampleswere charged to 700 volts. The reason for this voltage difference wasthat the 1:1 molar sample was a slightly thinner sample.

After each exposure, the voltage change (Av.) in the exposed step areaswas measured with a feedback electrostatic voltmeter manufactured byMonroe Electronics. These measurements were then used to prepare thegraphs shown in FIGS. 6, 7, and 8, in which the diffuse transmissiondensity of the step wedge is plotted versus voltage. FIG. '6 is thegraph for the measurements of the 1:1 molar sample. FIG. 7 is the graphfor the measurements of the 40:1 molar sample. FIG. 8 is the graph forthe measurements of the 150.1 molar sample.

By comparing the 12.7 mm. exposure setting curves of FIGS. 6 and 8, itcan be quickly seen that the slope of the FIG. 8 curve is much steeperthan that of the FIG. 6 curve. This is an immediate indication that thevoltage change (Av.) is greater with the 150:1 molar sample or with aweakly absorbing photoconductive material. As is well known inelectrophotography, the greater the voltage change -(Av.), the greatercapability of developing a high image quality. To further illustratethis comparison, the following table has been prepared from these graphsfor the image transmission densities of 0.2 and 0.4.

Voltage Change (Av.) in volts For 0.2 image transmission For 0.4 imagetransmmm.

1:1 molar: 20

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that variations in form may be made thereinwithout departing from the spirit and scope of the invention. Forexample, the contact exposure method and member of the present inventionmay be employed in persistent electrophotographic methods, such as thatdisclosed in US. Pat. 2,845,- 348 or any other method where thephotoconductor is exposed b'efore charging. Also, the contact exposuremethod and member of the present invention can be used in conjunctionwith the charge transfer technique disclosed in US. Pat. 2,825,814.

In addition, while the term opaque document is understandable in thecontext of the foregoing specification, to make certain that there is nomisinterpretation of this term, opaque document is a document permittingless transmisson of light than that of a vellum document, such as bondpaper. The term opaque document, however, does not mean one throughwhich no visible light can be transmitted.

What is claimed is:

1. In an electrophotographic methodof forming an electrostatic chargepattern on a photoconductive member, the stepscomprising:

bringing an opaque document into face-to-face contact with aphotoconductive member comprising a photoconductive layer and a lightreflecting backing having a reflectivity greater than 0.80, thereflectivity of the surface of the-document being such that thereflectivity product of the light reflective backing and the surface ofthe document is greater than 0.25, and

exposing the photoconductive member through the back of the document towavelength of light only weakly absorbed by the photoconductive layersuch that less than 30% of the total exposing light is absorbed during asingle passage through the photoconductive layer whereby the light isreflected back and forth between said highly light reflecting backingand the nonimage face surface of said document, with a portion of thelight being absorbed during each passage through the photoconductivefilm.

2. The method of claim 1 wherein the document is translucent and isbacked by an opaque sheet. I

3. The method of claim 1 wherein the reflectivity product is greaterthan 0.70.

4. The method of claim 1 wherein the photoconductive layer is a nonlightscattering material.

5. The method of claim 4 wherein the photoconductive layer is an organicphotoconductor.

References Cited UNITED STATES PATENTS Snelling 96- 1 GEORGE F. LESMES,Primary Examiner J. C. COOPER HI, Assistant Examiner r U.s.*c1. X.R.96-15; 117-175

